This invention relates to springs, particularly to garter springs.
A garter spring is annular. By definition, a garter spring is enclosed on itself, so that the spring is a resilient band having continuous tension. Such springs are used for a variety of applications.
Several methods of making garter springs are currently in practice, each of which has certain disadvantages. In one method, one end of a straight spring is screwed inside the other end (see, for example, Mason, U.S. Pat. No. 863,034). This threading method requires the spring to be formed so that the outside diameter of the coils at one end is smaller than at the other end. The outside diameter of the coils from the smaller end, however, must be greater than the inside diameter of the coils from the larger end to make the joint secure. A secure connection requires at least three coils from each end.
This threading method is fraught with problems. First, the method of connection causes an inaccuracy because it expands the outside diameter of the spring at the joint. Second, since a secure connection requires at least three coils from each end, the spring properties of at least six coils are destroyed. Third, since it takes at least three unwinding turns to screw together the minimum of three coils, shorter springs will be permanently distorted. Fourth, vibrations or handling may cause the ends to unscrew.
Another variation of the threading method is taught by Becker, U.S. Pat. No. 3,276,761. In Becker's patent, one coil of the inserted smaller end projects outwardly to fit between coils of the larger end. This method improves only slightly over the prior art, as it relies on the projecting coil to hold the spring in place. Moreover, it requires coils to be prestressed, with variations in the extent to which coils are to be prestressed.
Another variation is taught by Skinner, U.S. Pat. No. 3,186,701. Skinner's patent requires the relatively difficult task of grinding the outer periphery of each coil of the spring. This method involves a lot of work, weakens the spring, and may be inappropriate for certain applications.
Another method of forming an annular spring is to weld together the tip ends of a straight spring. This method was employed by Mather, U.S. Pat. No. 3,468,527. In this method, the point of attachment is only as strong as the small weld between the tips. The tips may also be difficult to line up for this procedure, leaving the pitch between the coils at the point of attachment different from the rest of the spring. Additionally, Mather's spring requires variations in pitch and can only be used for loading in a predetermined direction. Garter springs, however, must often be used where the direction of the load is not predetermined.
Another method disclosed in the prior art requires use of a tube-like coupling. To form a garter spring using this method, each end of a straight spring is threaded through the tube-like coupling. (Jack, U.S. Pat. No. 3,190,633). This method may be impractical, because it requires close threading of relatively small parts.